For my gas engine 4.3l V6, I choose moly (moly, not chrome moly). I did alot of reading before I chose those, and I can't say I've heard anything about them being difficult to seat. I've read they seat much much quicker and better, and have better oil retention and have higher resistance to scuff. I did read you can screw them up more easily on installation (they chip easily if you catch an edge going in the bore), and if you don't follow the proper break in procedure for them, they can get ruined or your cylinders glazed. The break-in procedure I read for moly is on first start, keep it at 2000rpm until operating temp, that's normal, time it, carb adjustments, then go out on the road and do several 1/2-3/4 throttle line launches and at speed launches. After that, so I've read, they are pretty much seated with varying throttle driving after 500-1000mi. Let it idle too much less than 100mi, or sustained cruising at any speed, and you'll do 'em fairly quickly. I've read cast rings can take up 30,000mi to seat.
Just what I've read in various places, mostly books, some internet articles and some internet forums. So take it at face value.
My little v6 is running great at 1400mi, dyno pulls show 242hp@4800 and 259ft'lbs at 3750. I didn't go wild either, just 1 up cam from stock and vortec heads from a 2004, edelbrok manifold and holley 390cfm carb. Getting 22mpg already in the C10 truck it's in, and no blue smoke.
I never did any reasearch on rings for diesel engines, so can't really say if moly is a good choice there or not.
